The invention relates generally to apparatus and methods for controlling the flow of fluids. More particularly, the invention provides a valveless pump of simple construction, and which may be made quite small using micromachining techniques. A pump according to the invention may use internal elements such as airfoil-shaped structures as direction-sensitive elements for producing different drag forces as fluid flows through the micropump in different directions.
Conventional pump designs typically use valves as flow directing elements. These valves allow fluid to flow from the low pressure end to the high pressure end of the pump, and to prohibit flow of the fluid back from the high pressure end to the low pressure end. Several types of valves are used in practice. Passive valves may employ an object such as a movable plate as a direction-checking component. The plate opens due to a pressure difference when fluid is pumped forward, and then closes to prevent fluid flowing backward when the pressure is reversed. Such passive valves are popular in many engineering applications.
Certain drawbacks limit the application of such valves in micropump designs. To begin with, it is not easy to micromachine the micro-dimensioned moving parts that such valves require. Secondly, the actions of the moving parts, such as the opening and closing of the plate, may damage cells within bio-fluids or other fragile substances. Thirdly, when the working fluid includes particles, the valve may become blocked by a collection of those particles between the moving elements. Finally, the continuous opening and closing action may lead to fatigue in the valves and failure of the micropump.
Active valves have similar drawbacks, but provide greater freedom for control of the fluid delivery, and less backflow. Active valves are even more difficult to fabricate, though, because of the greater complexity of the moving parts and other related structures.
Valveless micropumps or fixed valve micropumps have been devised and are finding increasing application, especially in bio-engineering applications. There are several advantages in valveless micropumps. Firstly, the valveless micropumps are much easier to fabricate using standard micro-machining techniques. Secondly, valveless micropumps are more reliable because there are no moving elements in the inlet and outlet channels. Thirdly, the valveless micropumps, unlike other pump designs, do not have any moving components in the inlet and outlet channels, and therefore will not cause much damage to bio-molecules. Also malfunctions due to blockages are minimized.
It is known in the art to provide a fixed valve conduit in which the design of the conduit is flow-direction sensitive. A lower drag force is produced when fluid flows in a forward direction than when the fluid is flowing in a backward direction. Such designs may be based on the concept of non-unit drag ratio of the backward flow to the forward flow. The efficiency of the one-directional flow conduit can be measured by such ratio. The larger the ratio, the more effective the valving action of the conduit.
It is also known in the art to provide a micropump having fixed valves fabricated using micromachining techniques. Again, the design thereof can be based on the concept of differentiated drag between the forward and backward flows.
Other work has been directed toward the aerodynamic characteristics of airfoils. Lift and drag forces have been measured for different angles of attack of airfoils from zero to 180 degrees. Airfoils have been shown to have different drag values for fluid flows arriving from different directions. The following table lists measured drag coefficients Cd for various angles of attack a:
TABLE 1a051015. . .165170175180C0.0100.0140.0180.190. . .0.2300.1400.0550.025d30800000Cd is defined by:                     Cd        =                  Drag                      1            ⁢                          /                        ⁢            2            ⁢            ρ            ⁢                                                   ⁢                          gV              2                                                          Eq        .                                   ⁢        1            where Drag is the drag force caused by the flow; ρ is the density of the working fluid; g is the gravitational force and V is the flow velocity.
From Table 1, the drag ratios between the forward and backward flow may be obtained (from opposite directions). This ratio, η, is also known as the drag efficiency and is defined by:                     η        =                              Cd                          180              -              a                                            Cd            a                                              Eq        .                                   ⁢        2            Table 2 gives the η ratios for a ranging from 0 degrees to 15 degrees, based on Table 1 and Equation 2.
TABLE 2Drag efficiency at Reynolds number 160,000a051015. . .n2.42723.92867.44681.6842. . .From Table 2, it can be clearly observed that airfoils can generate very high drag efficiency. This becomes obvious when it is noted that the airfoil exhibits its streamline-body characteristic property when the flow direction is from its leading edge to its trailing edge. In the reverse flow direction when the flow is from the trailing edge to the leading edge, the airflow no longer presents itself as a “streamline body” and shows non-streamline characteristics.
It would be desirable if an improved micropump could be devised to take advantage of advances in knowledge regarding the behavior of airfoils in moving fluids. Such a micropump should be reliable, efficient, of simple construction, and feasible to fabricate using known micromachining techniques. These and other advantages are provided by the novel apparatus and methods described herein.